The invention relates to an apparatus for measuring the static charge developed by a flowable solid as it passes through a conduit. The apparatus comprises a receptacle for receiving a sample of a flowable solid, a conduit for conveying the flowable solid and a device for measuring the static charge of the flowable solid as it exits the conduit. A method for measuring the static charge of a flowable solid is also provided.
A common problem in industry has been the processing and handling of flowable solids. Because of their high surface area-to-mass ratio and other factors, they have a tendency to develop electrostatic charges, particularly when they are conveyed through pipes or other conduits. The electrostatic charges cause the flowable solids to collect on contiguous surfaces. This phenomenon reduces the efficiency and accuracy of their conveyance. On occasion, charges can develop to an extent that spontaneous electrical discharges can occur. The discharges can lead to damage of equipment and storage containers as well as injury to personnel. Testing of the propensity of a flowable solid to develop a static charge as well as the nature of the charge developed can lead to the development of systems to improve the conveyance of flowable solids without the development of static charges.
The principal cause for static charge generation when a flowable solid is conveyed through a conduit is frictional contact of dissimilar materials by a physical process known as the triboelectric effect. As the flowable solid passes through the conduit, static is generated by frictional contact between the flowable solid and the walls of the conduit. The frictional contact causes a flow of electrical charges from the walls of the conduit to or from the flowable solid.
Static charges also present problems in olefin polymerization processes. It is well known that in fluidized bed polymerization reactors, static charges often develop as a result of frictional contact between the growing polymer particles and the metal walls of the reactor. Static charges also develop on the dry catalyst particles as they are injected into the reactor. The catalyst is normally injected using nitrogen or other inert gases to convey the catalyst through a small stainless steel tube into the reactor. As the catalyst passes through the tube, friction between the catalyst and the wall of the tube generates a charge on the catalyst, which then influences the static charge on the fluidized bed.
Static charges in a fluidized bed reactor are known to contribute to a phenomenon known as sheeting. By xe2x80x9csheetingxe2x80x9d is meant the adherence of fused catalyst and resin particles to the walls of the reactor. The sheets will eventually dislodge from the wall and, if the sheets are large enough, they can result in reactor plugging. A related phenomenon called xe2x80x9cdroolingxe2x80x9d or dome sheeting occurs when sheets of molten polymer form on the reactor walls, usually in the expanded section or xe2x80x9cdomexe2x80x9d of the reactor, and flow along the walls of the reactor and accumulate at the base of the reactor. This can result in plugging of the distributor plate in the reactor and loss of fluidization.
By knowing the size and polarity of the charge that a catalyst particle develops during injection, steps can be taken to neutralize the charge before sheeting occurs. One method of controlling the charge is through the use of static control agents. Timely addition of an effective amount of these agents has proven to be effective in reducing sheeting. Thus it would be helpful to know the nature of the static charge that a catalyst develops when it is injected into a reactor.
Some systems have been developed to measure static charges of flowable solids. Some of these involve the testing of the flowable solids themselves before they are introduced into a conduit or conveyance system. One such system is described in Ruckdeschel et al., Contact Electrification Between Insulators: Phenomenological Aspects, 25 Jol. Applied Phys. 4415 (1975). In this article the authors placed a sample of a flowable solid in a cylindrical Faraday cage. The cage was then rolled, and the charge developed by the flowable solid as it rolled inside the cage was measured. Another method involved firing a sample of polymer with a pneumatic gun, with the polymer striking a metal plate. Yamamoto, Triboelectric Charging of Polymer Particles by Input, 3 Part. Charact. 117-121 (1986). The deflected polymer was captured in a Faraday drum and the charge on the particle was measured. Neither of these systems duplicates the environment actually encountered by a flowable solid when it is conveyed through a conduit.
The study of electrostatic charges of flowable solids has been done with devices, which involve the use of moving parts. See, e.g., U.S. Pat. Nos. 2,421,430; 3,225,299; 3,406,344; 3,544,889; 3,727,125; and 3,943,437. Peterson, Contact Charging Between Nonconductors and Metal, 25 Jol. App. Phys. 907-915 (1954). Chubb et al., xe2x80x9cComparative Studies on Methods of Charge Decay Measurement,xe2x80x9d 50 Jol. Electrostatics 273 (1993).
Similarly, work has been done to measure the static charge of flowable solids using complex detectors or conveying mechanisms. These include the use of a photographic representation of the charge on a flowable solid, Kunkel, The Static Electrification of Dust Particles on Dispersion Into a Cloud, 121 Jol. App. Phys. 820-832 (1988).
Efforts have been made to develop simpler devices; however, the efforts do not measure the static charge developed on a flowable solid as it moves through a conduit. The work to date has involved the use of a by-pass line, U.S. Pat. No. 4,309,661; a deflecting metallic screen which is placed between the flowable solid and a static probe, See U.S. Pat. No. 5,541,518; and by measuring the static charge on the effected tube itself. Other devices are directed to using the static charge to determine other parameters such as solids density in the conveyance stream. See, e.g., U.S. Pat. Nos. 4,063,153; 4,074,184; 4,291,273; 4,082,994; 4,619,145; 4,714,890; 4,904,944; 4,607,228; and 5,022,274.
A need still exists for a simple device that can directly measure the static charge that a flowable solid, such as a supported catalyst, develops as it passes through a conduit.
The apparatus of the present invention comprises a receptacle for receiving a sample of a flowable solid, a conduit for conveying a flowable solid and a device for measuring the static charge of the flowable solid as it leaves the conduit. Additionally, a pressurized gas delivery system can be used to help convey the flowable solid through the conduit, thereby simulating actual commercial conditions.
In practice, a sample of a flowable solid to be tested is placed in a receptacle at one end of a conduit. The flowable solid is then conveyed through the conduit in such a way that static charge develops on the flowable solid. Usually, this involves conveying the flowable solid through the conduit using an inert pressurized gas. The conduit should be of sufficient length and diameter to promote the development of a static charge on the flowable solid. Additionally, the conduit should be constructed of the same materials as the conduit being modeled. As the flowable solid exits the conduit, it enters a device capable of measuring the static charge on the flowable solid. The preferred device is a Faraday drum of the type described below.
The Faraday drum represents an application of a fundamental measurement technique named after Michael Faraday, a British physicist and chemist (1791-1867). The system operates on the principal of displaced current. Because the inner drum of the Faraday system is connected to ground through a low resistance current measuring device, the voltage on the inner drum must be zero. When charged flowable solid enters the Faraday drum, an electrical charge is displaced from the Faraday drum through a current measuring device to ground, as required to keep the overall system voltage zero. The amount of charge displaced to ground is equal and opposite to the charge that enters the drum with the resin and catalyst particles. The current flow is recorded and integrated over the time period that the particles are introduced to the system to provide a quantitative measurement of the charge.
In practice, the actual measurement taken is the current that flows from ground to the drum, which is opposite in sign to the current that flows from the drum to ground. In effect, the current meter is connected backwards to provide a sign inversion to the measurement. In this way, the measured current, integrated over time, is exactly equal to the charge that enters the system, both in magnitude and sign.
The preferred means of measuring the current flow is with an electrometer, such as a Keithley Model 610C. These devices provide the dual capability of measuring and integrating the current as required by the Faraday technique. Other devices to measure and integrate the current can be employed, as known by those skilled in the art. For example, the current can be measured with a simple current meter (ammeter), with the readings recorded and digitally integrated over time to provide the required charge measurement.
There is also provided a method for determining the static charge developed by a flowable solid as it is conveyed through a grounded conduit. The method involves introducing a sample of flowable solid into one end of a conduit, conveying the polymer sample through the conduit in a manner sufficient to promote the development of a static charge on the flowable solid and measuring that static charge on the flowable solid as it exits the conduit. The method may also involve the introduction of a gas into the conduit to help convey the flowable solid through the conduit.
The apparatus and method of the invention provide a relatively simple and direct method for measuring that static charge developed by a flowable solid when it passes through a conduit. Information developed using this device and method can be used to design systems to prevent static charge development, as well as a means for monitoring static charge development in commercial systems.